There is a major need for a system which will automatically expose and handle double-sided printed circuit boards, especially such boards having large areas. For example, each board may be 18.times.24 inches in size, although much larger sizes may also be handled. The "boards" may be either relatively rigid (with typical thicknesses in the range of 0.04 inch to 0.1 inch), or they may be multilayer boards as thin as 0.005 inch.
Each side of each board is made sensitive to ultraviolet light, so that it may receive an image corresponding to that of artwork, there being one artwork (actually, a photomask) for one side and another artwork for the other side. It is extremely important that there be precise registry between the artwork and the board, so that the resulting images produced by masking and exposure will register on opposite sides of the board. Then, connections may be made through the board to registered portions of the electric circuits. The artwork is often extremely fine, so as to be difficult or even impossible to see with the naked eye.
It is known in the art to position circuit boards by the cooperative action of pins and of slots in the circuit board. Two of the circuit-board slots are oriented parallel to each other, while a third is oriented perpendicular to the two parallel slots, being disposed between such parallel slots and spaced from each. Pins having accurately-located positions, and having diameters matched to the widths of the slots, are then inserted through the slots in order to position the circuit board in a desired location.
The indicated prior-art approach has substantial limitations relative to precision and other factors. This is because the slot widths cannot be known with great precision, being instead within certain tolerance values. When, because of tolerances, a pin is somewhat loose in its slot, the degree of precision of circuit-board location is reduced. On the other hand, when a pin is tight in its slot, movement of the pin in a direction perpendicular to the circuit board tends to move the board in the direction of pin motion, and also tends to enlarge the slot in the region of the pin. When a pin is sufficiently large to locate the board with extreme accuracy, it is also, inherently, sufficiently large to tend to move the board when the pin is introduced into the slot.
It is to be understood that the widths of the slots may vary with different circuit-board materials, even when the same punch is used to form the slots. The widths of the slots may also vary even when the slots are formed by routing instead of punching.
The above-indicated problems are exacerbated when the board is floating on a layer of air, and does not have anything thereover to tend to hold it down (hold-down means being undesirable in that they may tend to scuff the upper side of the board). When the board is floating on air, the upward dragging action created between upwardly-moving tight pins and the walls of the slots creates a distinct undesirable lifting of the board. It is also to be understood that effecting floating of the board on air is important in order to prevent scuffing of the board when it is brought into the desired precision location due to operation of the pins. In other words, there should be no sliding of the board on a flat surface, even though the relatively small distance necessitated by the registration-creating action.
In accordance with the present invention, locater elements are inserted through slots in circuit boards in such manner as to reduce to an absolute minimum the tendencies of such elements to drag against and thus lift the board. Furthermore, and very importantly, the present invention eliminates the problem of tolerance variations in the widths of the slots. In addition, the present locater elements do not tend to bite into the slot walls, and are such as to permit downward movement of the circuit board, to a clamped position preparatory to exposure, without detracting from the registry and again without injury to the slots.
The present locater elements, and actuation-apparatus therefor, are employed in combination with air-flotation and vacuum apparatus which achieve scuff prevention while at the same time assuring that the circuit board during exposure will be--at all portions thereof--clamped firmly.